PrepTest 151, Section 2, Question 16
Scientist: An orbiting spacecraft detected a short-term spike(64 more words not shown)
Which one of the following,(7 more words not shown)
- AConditions on Venus(14 more words not shown)
- BEvidence suggests that(14 more words not shown)
- CLevels of sulfur(15 more words not shown)
- DTraces of the(18 more words not shown)
- EMost instances of(15 more words not shown)
Reading the argument
A volcano could explain the spike. A chemical cycle could explain it too. What lets us choose between them?
The comparison with Earth makes a volcanic explanation plausible, especially since Venus shows signs of past volcanic activity. Still, that doesn't establish what caused this particular spike. The scientist urges us not to conclude that a volcano caused it, pointing to the lack of identified active volcanoes and the possibility of atmospheric cycles.
What we’re looking for
I'll accept that caution without treating the alternative as proven. Knowing that atmospheres undergo cycles doesn't show that a cycle produced this event. To weaken the argument, I'd look for something favoring volcanic activity or undermining a reason the scientist gives for withholding that conclusion.
Working the answers
- ACorrect
Now the absence of an identified eruption means much less. If conditions make direct detection unlikely, a volcano could erupt without our instruments recognizing the eruption. We shouldn't give the lack of a detection the same weight we would give it if our instruments were likely to spot any eruption.
A doesn't prove that a volcano caused the sulfur dioxide spike. It weakens the scientist's use of non-detection as a reason to doubt that explanation. The possible chemical cycle remains, but one important piece of the argument has lost force.
- B
An earlier spike tells us that something similar happened before, but not what caused either event. It could have been another eruption or part of a recurring atmospheric process. Indeed, recurrence could fit the scientist's cyclical alternative. Without information about the earlier spike's cause, B doesn't favor the volcanic explanation over the scientist's reasons for caution.
- C
The issue is a short-term increase on Venus, not whether Venus usually has more sulfur dioxide than Earth. A higher long-term background level doesn't explain why the concentration suddenly rose. We still need evidence about the source of that change.
- D
This concerns how long traces remain after an eruption on Earth. It doesn't identify the source of a short-term spike on Venus or show that Venus's atmosphere behaves the same way. Lingering traces of an old eruption are also different from evidence explaining a newly observed increase. D doesn't establish that a volcanic event produced this spike or undermine the scientist's non-detection evidence.
- E
Another source of spikes on Earth doesn't establish the source on Venus. We aren't told that fossil-fuel burning is occurring there. And introducing another possible nonvolcanic explanation would not, by itself, undermine the scientist's caution about concluding that a volcano caused this event.
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